In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method which comprises rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink unreceptive area), thereby making a difference in adherence of ink on the surface of the lithographic printing plate, and depositing the ink only on the image area by utilizing the nature of water and printing ink to repel with each other, and then transferring the ink to a printing material, for example, paper.
In order to prepare the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (also referred to as a photosensitive layer or an image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by a plate making method comprising exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer in the portion for forming the image area, removing the unnecessary image-recording layer other than the image area by dissolving with an alkaline developer or an organic solvent thereby revealing the hydrophilic surface of support to form the non-image area.
In recent years, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor with the light thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
Also, in the plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary portion of the image-recording layer by dissolving with a developer or the like is required. Heretofore, the development processing process comprises three steps of developing with an aqueous strong alkali solution having pH exceeding 11, washing of the alkali agent with a water washing bath and then treating with a gum solution mainly comprising a hydrophilic resin. Therefore, the development system with the alkali agent has various problems in view of environment and running cost, for example, in that an automatic development processor per se requires a large space, in that it is necessary to provide a replenishment apparatus for supplementing decrease in the pH caused by absorption of carbon dioxide, and in that the load on disposal of a large amount of waste liquid, for example, development waste liquid, water washing waste liquid or gum waste liquid is large.
Particularly, since disposal of waste liquid discharged accompanying the development processing has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the decrease in alkali concentration of developer and the simplification of processing step have been further strongly required from aspects of the safety, consideration for global environment, space saving and low running cost.
Thus, as a method of preparing a lithographic printing plate using an alkali developer having relatively low pH, a method of developing a photosensitive lithographic printing plate with a developer having pH from 10.0 to 12.5 and containing an inorganic alkali agent and a nonionic surfactant containing a polyoxyalkylene ether group is proposed in Patent Document 1. However, this method has a problem in that an image-formation can not be sufficiently performed when the pH of developer is lower than 10.0.
Also, it is ordinarily difficult to ensure development property in the development processing in a range from acidity, neutrality to low alkalinity. Moreover, the component of photosensitive layer in the non-image area once removed is not easy to be dispersed in the developer. In particular, the component included in the polymerizable photosensitive layer, especially, the polymerizable compound is difficult to dissolve or disperse in the developer. Accordingly, when the development processing of lithographic printing plate precursor is repeatedly conducted, development scum derived from the polymerizable compound floats or accumulates in a developing bath not only to contaminate the development processor but also to adhere onto the lithographic printing plate precursor during the development processing, thereby causing a problem in that when the resulting lithographic printing plate is used for printing, printing satin or image defect may be apt to occur.
In response to the problem, a method of preparing a lithographic printing plate using a developer having pH from 2 to 10 and containing an amphoteric surfactant is proposed (see Patent Documents 2 to 4). However, this method is insufficient in the development property and does not solve the problem of the occurrence of printing stain.